From a H&S viewpoint we would probably have to exceed the estimated breaking strain by a considerable margin.
If, for example, it was thought that a 2" neolithic rope would have been up to the job, then we should use a 2" modern rope whose breaking strain will be considerably more, thus satisfying H&S, but its handling characteristics should be somewhat similar. I think this would be better than using a 1" rope with a similar breaking strain.